Rupture Directivity of Foreshocks Show Rapid Migration around Patches on a Complex Fault during the 2008 Mogul, NV Earthquake Sequence

An accelerating cascade of ruptures in the days leading up the 2008 M_w4.9 Mogul, NV earthquake highlight complex, but organized, spatiotemporal patterns of seismicity and stress drop on the mainshock fault. Previous work using empirical Green's function (EGF) analysis indicated nearly two orders of magnitude of variation of stress drops over a small spatial area on the fault plane. The EGF-derived rupture areas encircle distinct seismicity voids, previously interpreted as "patches" of heterogeneous stress on a doubly-kinked fault plane. High stress drop foreshocks nucleate at the edges of these "patches" or near changes in the strike and/or dip of the fault, while low stress drop foreshocks exist primarily outside the main seismicity void and no aftershocks re-rupture it. New 3D rupture directivity analyses of 87 of the earthquakes, 36 occurring before the mainshock, further illustrate the propagation of events around the "patches" leading up to the mainshock. We measure azimuthal variation in duration of the EGF-derived source time functions for events as low as M_L2.4 using a stretching- and correlation-based method. Then we perform a grid search to find the orientation and rupture velocity of the best-fitting line source. Combining robust directivity results with well-resolved seismicity locations and focal mechanisms enables a four-dimensional analysis of event triggering and interaction with respect to source physics. Our detailed analysis suggests that fault plane complexity, both geometry and proposed stress heterogeneity along the fault, plays a critical role in controlling the locations and migrations of seismicity, the stress drops of earthquakes, and the rupture direction of events within this energetic, shallow foreshock-mainshock-aftershock sequence. Of note, the M_w4.9 earthquake produced strong ground motions in excess of 1g, was preceded by over 60 days of foreshock activity, and was followed by aseismic slip equivalent to the moment release of the mainshock itself.